1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system with an improved focusing method.
2. Description of the Prior Art
In a typical conventional zoom lens system, the same lens unit is used to perform focusing in the entire focal length range of zooming. However, usually, different lens units are the most suitable for moving for focusing in different focal length ranges. Therefore, to enable a closer photography while maintaining more excellent close photography performance, it is desirable to use, for focusing, different lens units in different focal length ranges.
In view of this, U.S. Pat. No. 5,042,926 discloses a three unit zoom lens system of negative, positive, negative configuration where the second lens unit is used to perform focusing in a focal length range (30 mm to 36 mm) from the shortest to the middle focal length conditions and the first lens unit is used to perform focusing in a focal length range (36 mm to 105 mm) from the middle to the longest focal length conditions. Also, a two-unit zoom lens system of positive, negative configuration is known as a product where all the lens units are moved out to perform focusing in the focal length range from the shortest to the middle focal length conditions and the first lens unit is moved out to perform focusing in the focal length range from the middle to the longest focal length conditions.
However, the former lens system presents a problem that the back focal length is extremely short in the focal length range from the shortest to the middle focal length conditions. If the back focal length is too short, the lens diameter increases, so that the size of the lens system increases. In the latter lens system where all the lens units are moved out to perform focusing, since the weight of the moved lenses is great, the size of the lens system increases due to a restriction on the mechanism for moving the lens units.